pass the parameter environment to traits::find_associated_item

dropping the param-env on the floor is obviously the wrong thing to do.

The ICE was probably exposed by #54490 adding the problem-exposing use of
`traits::find_associated_item`.

Fixes #55380.

src/test/run-pass/issue-55380.rs

@@ -0,0 +1,38 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-pass
+
+#![feature(specialization)]
+
+pub trait Foo {
+    fn abc() -> u32;
+    fn def() -> u32;
+}
+
+pub trait Marker {}
+
+impl Marker for () {}
+
+impl<T> Foo for T {
+    default fn abc() -> u32 { 16 }
+    default fn def() -> u32 { 42 }
+}
+
+impl<T: Marker> Foo for T {
+    fn def() -> u32 {
+        Self::abc()
+    }
+}
+
+fn main() {
+   assert_eq!(<()>::def(), 16);
+   assert_eq!(<i32>::def(), 42);
+}